A dental appliance is made on an artificial stone cast of the mouth. A primary impression must be made of the mouth in order to produce the artificial stone cast. The technical quality of a dental appliance is in direct proportion to the accuracy of the primary impression, the correct clinical procedure and the accurate coverage of the teeth and pertinent soft tissues with the impression material.
Present techniques for forming dental impressions in fully or partially dentulous mouths are generally divided into two categories: The first is a "primary" impression and employs a stock dental tray filled with soft impression material, usually a dental alginate. The soft material is carried into the patient's mouth by the impression tray. When positioned or centered over the teeth and other areas of interest, the soft material is pushed over the dental structures by the application of vertical pressure on the tray. This carries the soft material over the teeth and related tissues in the dental arch. The impression material sets in a few minutes after mixing and is transformed into a flexible, solid mass which no longer flows. The flexibility of the impression material allows its removal from the undercuts formed by the bulging surfaces of the teeth and related tissues.
This "primary" impression is used to form a cast of artificial stone, which is a positive reproduction of the dental arch and oral tissues recorded in this "primary" impression. The disadvantages of using a case made from the "primary" impression as a final working cast are: (1) insufficient coverage of the edentulous areas and faulty peripheral extensions of the dental arch; and (2), lack of perfect detail caused by failure of the impression material to reach every portion of the teeth, the gingival crevices and the occlusal sulci. This failure is caused by the entrapment of air as the impression material, being applied without sufficient pressure and proper direction of flow, is not forced into the deeper crevices and depressions.
This requires the implementation of a "secondary" impression which employs the use of a custom dental tray formed on the primary cast. A custom tray is made of hard material and is formed loosely to fit the teeth and tissues and is supposed to provide the necessary coverage of teeth and soft tissues, as well as establishing the proper extensions of the peripheral borders. The construction of a custom tray is a separate procedure accomplished between the first and second operative appointments. The patient must return a second time to the dental office so that another impression can be made, using the custom tray, for the final working cast on which the appliance is constructed. When employing a custom tray, a soft impression material is also used in the same manner as with the "primary" impression. However, a "secondary" impression of the mouth with a custom tray does not thoroughly correct the main disadvantage associated with the "primary" impression, i.e., air entrapment. The soft impression material which flows around the teeth and the soft tissues of the impression area, in either the "primary" or "secondary" impression, traps air in many of the critical areas of the impression. When the material sets and the impression is removed from the mouth, air voids appear on the surface of the impression. When the impression is poured with dental stone, these air voids in the impression show up on the final cast as stone bubbles and other imperfections. These must be mechanically carved away with a sharp instrument from critical surfaces before a dental appliance can be constructed. However, this often leaves an imperfect cast, and many times the correction is not possible, such as in an occlusal rest groove prepared by the operator. If the mechanical correction of these critical areas with a sharp instrument is not perfect, the appliance will not fit the teeth and will require additional time-consuming adjustments by the operator who tries to make it useful.
Some dentists attempt to prevent air voids in the impression by applying dabs of the impression material to the teeth with their index finger. Another method is to apply the impression material to the teeth with a syringe. Both of these procedures produce an imperfect application of the impression material and consumer critical time. This often results in the materials setting into its elastic form before the impression tray and the impression material can be inserted into the mouth and over the teeth, thus requiring a new impression. Also, these methods do not totally alleviate the problem of imperfections in the cast caused by air voids. An impression free of air voids is needed to provide an accurate cast for the construction of a removable appliance which will be comfortable to the patient without the necessity of mechanically correcting the cast and grinding the insides of the metal clasps in order to make the appliance fit.